The present invention relates to a solid state image sensor for use in picture information processing devices such as facsimile transmitter/receivers, character readers and television.
Various image sensing devices are presently known which are useful in picture information processing devices. These include charge transfer devices, such as CCD's and BBD's, and XY address devices, such as one-dimensional line sensors and two-dimensional area sensors. Each of these known devices, however, suffers from a common drawback with regard to accurately dealing with variations in illuminance caused by shading, colors, or differences in paper quality.
A typical image sensor according to the prior art contains an array of photoelectric devices, each of which represents a picture element or pixel in the reproduced image. The illuminance from each region of the subject picture causes the corresponding photoelectric device to produce an output signal, a high illuminance leading to a high output signal. The output signal from each photoelectric device is then compared with some arbitrary threshold level, T to determine if the picture element should be black or white.
FIG. 1 shows graphically the output currents from a conventional image sensor of the line type. T is the threshold output level for distinguishing picture elements between black and white. Any region of the subject picture that causes the photoelectric device to produce an output current greater than T for example I.sub.11, gives rise to a white picture element. Any region of the subject picture having an illuminance that causes the photoelectric device to produce an output less than T for example I.sub.12, gives rise to a black picture element. Similar principles are used in conjunction with filters to generate color images.
From this description, it can be seen that the accuracy of reproduction for subject pictures having variable illuminance is troublesome. Picture regions which are gray are converted to either black or white, depending on where the threshold level is set, giving rise to loss of detail. This problem can also limit the utility of differing papers and inks, unless some provision is made to vary the threshold level to correct for differing reflectivities. Mechanisms are available to make this correction, but their use increases the cost and the time required for transmitting images.
It is the object of the present invention to provide image sensing devices which correctly represent regions of intermediate illuminance, thus preserving detail in the transmitted image and minimizing the need for correction mechanisms.